In situ and non-intrusive laser-based spectroscopic methods have been widely used for gas sensing and analysis in environmental monitoring and combustion diagnostics. Among various laser diagnostic methods, laser absorption spectroscopy (LAS) and laser dispersion spectroscopy (LDS) are two representative spectroscopic techniques that demonstrate accurate quantitative measurement and high sensitivity. In particular, LDS is a technique for gas sensing by detecting the optical phase signal associated with a refractive index change instead of measuring laser intensity attenuation as performed in a LAS technique. The LDS technique can overcome the baseline fitting and normalization problems found in a LAS technique. The LDS technique also has an intrinsic immunity to laser power fluctuations, has a large dynamic range, and allows for calibration-free operation.
A direct dispersion measurement can be realized using either chirped laser dispersion spectroscopy (CLaDs) or heterodyne phase-sensitive dispersion spectroscopy (HPSDS). CLaDs uses a frequency-chirped laser to transform an optical phase variation into a frequency shift, by which a dispersion spectra can be recovered. In comparison, HPSDS has the advantages of simpler optical configurations and data acquisition processes by intensity modulation of the lasers to generate spectral sidebands. Currently, several HPSDS-based gas sensors have been developed for trace gas sensing. For near-infrared HPSDS sensors, commercial electro-optical modulators (EOMs) and acousto-modulator (AOMs) are mostly used to modulate the laser intensity to generate a multi-color laser beam. Due to the commercial unavailability of EOMs and AOMs in the mid-infrared region, direct intensity-modulation of a laser injection current can be used in order to generate spectral sidebands. An accurate spectroscopic model is required to take into account the entire physical process from a mid-infrared laser emission with high-frequency current modulation, the light-gas interaction resulting in dispersion and absorption, to the final heterodyne phase detection.
Recent advancements in laser technology have provided room-temperature, high-powered laser sources in the mid-infrared region. Stronger absorption bands of combustion gases (i.e., H2O, CO2, CO, NO) are located in the mid-infrared region. As the dispersion associated with the refractive index is related to the frequency-dependent absorption coefficient via the Kramers-Kronig relation, the spectral feature with a stronger absorption is accompanied by a stronger dispersion.
Currently no research is reported on the development and application of LDS for combustion diagnostics. In practical laser-based combustion diagnostic systems, laser power fluctuations and photodetector (PD) drifts introduce inevitable measurement uncertainties. The intrinsic power fluctuation immunity and calibration-free operation characteristics of the methods and system described herein make the dispersion spectroscopy combined with heterodyne detection more suitable for diagnostics under harsh environments. Accordingly, embodiments of the subject invention provide a dispersion spectroscopy technique in the mid-infrared region and other spectral domains.